Conventionally there has been proposed a structure for a differential device mounted in an automobile or a similar vehicle. More specifically, a pinion shaft is inserted in a through hole provided in a peripheral wall of a differential casing and a pin is inserted in a pin insertion hole bored in the peripheral wall of the differential casing in a direction orthogonal to the through hole to engage the pin with the pinion shaft to fix the pinion shaft to the differential casing to prevent the pinion shaft from escaping (see Japanese Patent Laying-Open No. 2008-128440 (Patent Literature 1) for example).